Extendable vehicle steering shaft

ABSTRACT

In a telescopic shaft for vehicle steering, assembled in a steering shaft of a vehicle and including a male shaft and a female shaft so fitted as to be mutually incapable of rotating but mutually slidable, at least one set of torque transmission members are disposed in at least one set of accommodating portions formed in an outer peripheral surface of the male shaft and in an inner peripheral surface of the female shaft, and at least the one set of torque transmission members are cylindrical members that gradually decrease in their diameters toward end portions in an axial direction.

TECHNICAL FIELD

The present invention relates to a telescopic shaft for vehiclesteering, capable of actualizing a stable slide load and transmittingtorque in a high-rigidity state by surely preventing backlash.

BACKGROUND ARTS

FIG. 7 shows a general type of steering mechanism of an automobile. InFIG. 7, the symbols a and b represent telescopic shafts. The telescopicshaft a is constructed by spline-fitting a male shaft and a female shaftto each other. This type of telescopic shaft is, however, required tohave performance that absorbs an axis-directional displacement occurredwhen the automobile travels but transmits neither the displacement norvibrations onto a steering wheel. This performance is required generallyin such a structure that a vehicle body takes a sub-frame structure, aportion c for fixing an upper portion of the steering mechanism and aframe e to which a steering rack d is fixed are different units, and theframe e and the steering rack d are fixed by fastening through anelastic member f such as rubber interposed therebetween. Further, thereis other case in which a worker, when fastening a steering shaft joint gto a pinion shaft h, temporarily contracts the telescopic shaft andthereafter fits and fastens the joint g to the pinion shaft h, andtherefore a telescopic function is needed. Moreover, the telescopicshaft b provided in the upper portion of the steering mechanism is alsoconstructed by spline-fitting the male shaft and the female shaft toeach other. This type of telescopic shaft b is required to have afunction of shifting a position of a steering wheel i in the axialdirection in order to obtain an optimal position when a driver drivesthe car and adjusting this position, and hence an axis-directionaltelescoping function is requested of this shaft. In all cases describedabove, the telescopic shaft is requested to reduce backlash noises atthe spline-fitting portion, a feeling of backlash on the steering wheeland a slide resistance when sliding in the axial direction.

Such being the case, according to European Patent Application Laid-OpenPublication No. EP1078843A1, plural sets of torque transmission members(cylindrical members) are fitted in plural sets of axis-directionalgrooves formed in an outer peripheral surface of the male shaft and inan inner peripheral surface of the female shaft.

Each set of torque transmission members (the cylindrical members) areconstructed of a plurality of needle rollers arranged side by side inthe axial direction.

With this arrangement, when the torque is not transmitted, the backlashbetween the male shaft and the female shaft can be prevented, wherebythe male shaft and the female shaft can slide in the axial directionwith a stable slide load without any backlash. Further, whentransmitting the torque, the male shaft and the female shaft cantransmit the torque in a high-rigidity state by preventing the backlashin a rotating direction.

According to European Patent Application Laid-Open Publication No.EP1078843A1, however, each set of torque transmission members (thecylindrical members) are constructed of the plurality of needle rollersarranged side by side in the axial direction, and therefore, theassembly is complicated enough to take an excessive period of assemblingtime.

Accordingly, if the number of the needle rollers in each row is set toone or two, accuracy of straightness of each of the male shaft and thefemale shaft must be enhanced. This inevitably involves machining thegrooved portions and induces a rise in manufacturing cost. Hence,enhanced performance (which reduces the backlash in the peripheraldirection while restraining the slide resistance low) and a decrease incost can not be compatible.

Moreover, if the accuracy of straightness of each of the male shaft andthe female shaft is not enhanced, a high surface pressure might occur inthe vicinity of an end face of the needle roller when transmitting thetorque.

DISCLOSURE OF THE INVENTION

The present invention, which was devised under such circumstances, aimsat providing a telescopic shaft for vehicle steering, capable ofactualizing a stable slide load, transmitting torque in a high-rigiditystate by surely preventing backlash in a rotating direction and,besides, increasing a lifetime by decreasing both a period of assemblingtime of a torque transmission member and a surface pressure of thetorque transmission member while restraining the backlash in aperipheral direction. To accomplish the above object, in a telescopicshaft for vehicle steering, assembled in a steering shaft of a vehicleand including a male shaft and a female shaft so fitted as to bemutually incapable of rotating but mutually slidable, an improvement ischaracterized in that at least one set of torque transmission membersare disposed in at least one set of accommodating portions formed in anouter peripheral surface of the male shaft and in an inner peripheralsurface of the female shaft, and at least one set of torque transmissionmembers are cylindrical members that gradually decrease in theirdiameters toward end portions in an axial direction.

Thus, according to the present invention, at least one set of torquetransmission members are the cylindrical members that gradually decreasein their diameters toward the end portions in the axial direction, andhence the cylindrical members of which the number is as small aspossible can be disposed in one row of accommodating portions (theaxis-directional grooves). Accordingly, the assembling time can bereduced, and the cylindrical member can be manufactured at a low cost.Further, at least one set of torque transmission members are thecylindrical members that gradually decrease in their diameters towardthe end portions in the axial direction, and therefore the backlash inthe peripheral direction can be reduced while restraining the slideresistance low.

Moreover, in the telescopic shaft for vehicle steering according to thepresent invention, it is preferable that at least one set of torquetransmission members are cylindrical members subjected to crowning. Withthis construction, at least one set of torque transmission members arethe cylindrical members subjected to the crowning, and therefore thecylindrical members of which the number is as small as possible can bedisposed in one row of accommodating portions (the axis-directionalgrooves).

Accordingly, the assembling time can be reduced, and the cylindricalmember can be manufactured at the low cost.

The torque transmission member is the cylindrical member subjected tothe crowning, and hence the backlash in the peripheral direction can bereduced while restraining the slide resistance low.

Further, as the torque transmission member is the cylindrical membersubjected to the crowning, when the torque is loaded, a gentle surfacepressure is applied toward a central portion from the end portion of thecylindrical member, and a local surface pressure can be thus avoided, sothat a lifetime of the product can be kept long.

Moreover, in the telescopic shaft for vehicle steering according to thepresent invention, it is preferable at least one set of torquetransmission members are cylindrical members of which outside diametersvicinal to the end portions are worked in a tapered configuration. Withthis construction, when the torque is loaded, the gentle surfacepressure is applied toward the central portion from the end portion ofthe cylindrical member, and the local surface pressure can be thusavoided, so that the lifetime of the product can be kept long. In thetelescopic shaft for vehicle steering according to the presentinvention, it is preferable that the cylindrical member is a needleroller.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view of a telescopic shaft of vehiclesteering according to an embodiment of the present invention;

FIG. 2 is a cross-sectional view taken along the line X-X in FIG. 1;

FIG. 3 is a cross-sectional view taken along the line Y-Y in FIG. 1;

FIG. 4 is a perspective view of an elastic member (plate spring)connected by a connecting portion;

FIG. 5 is a view in the direction of an arrow A in FIG. 1;

FIG. 6 is a side view of a cylindrical member (needle roller) accordingto the embodiment of the present invention; and

FIG. 7 is a side view of a general type of steering mechanism of anautomobile.

BEST MODE FOR CARRYING OUT THE INVENTION

A telescopic shaft for vehicle steering according to an embodiment ofthe present invention will hereinafter be described with reference tothe drawings.

FIG. 1 is a longitudinal sectional view showing the telescopic shaft forthe vehicle steering according to the embodiment of the presentinvention. FIG. 2 is a cross-sectional view taken along the line X-X inFIG. 1.

As illustrated in FIG. 1, the telescopic shaft for the vehicle steering(which will hereinafter simply be termed the telescopic shaft) isconstructed of a male shaft 1 and a female shaft 2 that are so fitted asto be mutually incapable of rotating but mutually slidable.

As shown in FIG. 2, an outer peripheral surface of the male shaft 1 isformed with three lines of grooves 3 equally disposed at an interval of120 degrees in a peripheral direction and extending in an axialdirection. Each groove 3, as apparent in FIG. 2, in its cross-sectionalconfiguration, has a flat bottom portion 3 a at the center and flat sideportions 3 b, 3 b inclined divergently extending from both ends of thebottom portion 3 a toward the outside-diametrical sides. Further, theouter peripheral surface of this male shaft 1 is formed with three linesof grooves 4 taking substantially a circular-arc shape in section,disposed equally at an interval of 120 degrees in the peripheraldirection and extending in the axial direction in periphery-directionalareas between these three grooves 3 extending in the axial direction.

An inner peripheral surface of the female shaft 2 is formed with threelines of grooves 5 taking substantially a circular-arc shape in section,disposed equally at the interval of 120 degrees in the peripheraldirection and extending in the axial direction. Moreover, the innerperipheral surface of this female shaft 2 is formed with three lines ofgrooves 6 taking substantially a circular-arc shape, disposed equally atthe interval of 120 degrees in the peripheral direction and extending inthe axial direction in periphery-directional areas between these threegrooves 5 extending in the axial direction.

The grooves 3, 5 extending in the axial direction configure three setsof first accommodating portions for three sets of spherical members 7that will be explained later on, and the grooves 4, 6 extending in theaxial direction configure three sets of second accommodating portionsfor three sets of cylindrical members 8 that will hereinafter bedescribed. These three sets of axis-directional grooves 3, 5 (the firstaccommodating portions) and the three sets of axis-directional grooves4, 6 (the second accommodating portions), are arranged alternately inthe peripheral direction, wherein the grooves adjacent to each other aredisposed equally at an interval of 60 degrees in the peripheraldirection.

A first torque transmission device is constructed so that three sets offirst torque transmission members (spherical members) 7 for transmittingtorque in a way of their rolling when making relative movements in theaxial direction between the male shaft 1 and the female shaft 2 and oftheir being restricted by plate springs 9 when rotating, are soaccommodated as to be rollable between the three axis-directionalgrooves 3 of the male shaft 1 and the three axis-directional grooves 5of the female shaft 2 through three pieces of elastic members (the platesprings) 9 each taking a corrugated shape for pre-load. Each of thefirst torque transmission members 7 is constructed of a sphericalmember.

A second torque transmission device is constructed so that three sets ofsecond torque transmission members (cylindrical members) 8 fortransmitting the torque in a way that each permits the axis-directionalrelative movements of the male shaft 1 and the female shaft 2 andtransmits the torque when rotating, are so accommodated as to beslidable between the three grooves 4, extending in the axial direction,of the male shaft 1 and the three grooves 6, extending in the axialdirection of the female shaft 2. Each of the second torque transmissionmembers 8 is constructed of a cylindrical member extending in the axialdirection.

The plate spring 9 extends in the axial direction over substantially anentire length of the groove 3 within the groove 3 of the male shaft 1,and is provided between the spherical member 7 and the groove-formingsurface. The plate spring 9, when in a non-transmission state of thetorque, gives the pre-load to the spherical member 7 and to thecylindrical member 8 against the female shaft 2 to such a degree as notto cause backlash, and, when transmitting the torque, works to restrictthe spherical member 7 in the peripheral direction between the maleshaft 1 and the female shaft 2 in a way that makes its elasticdeformation.

In the thus-constructed telescopic shaft, the spherical members 7 andthe cylindrical members 8 are accommodated between the male shaft 1 andthe female shaft 2, and the plate springs 9 give the pre-load to thespherical members 7 and the cylindrical members 8 against the femaleshaft 2 to such a degree as not to cause the backlash. It is thereforepossible to surely prevent the backlash between the male shaft 1 and thefemale shaft 2 when transmitting none of the torque, and, when the maleshaft 1 and the female shaft 2 make the relative movement in the axialdirection, the male shaft 1 and the female shaft 2 can slide in theaxial direction with a stable slide load without causing the backlash.

Note that if the slide face is formed solely for sliding as by the priorart, the pre-load for preventing the backlash could only be retaineddown to a certain load. This is because the slide load is given bymultiplying a frictional coefficient by the pre-load, and, if thepre-load is increased in the anticipation that the prevention of thebacklash and rigidity of the telescopic shaft be improved, the slideload rises, thus falling into a vicious circle.

In this respect, the present embodiment adopts a rolling-based mechanismpartially, and hence the pre-load can be increased without inducing anoutstanding rise in the slide load. Both of the prevention of thebacklash and the improvement of the rigidity, which could not beattained by the prior arts, can be thereby attained without bringingabout the rise in the slide load.

When transmitting the torque, the three sets of plate springs 9 performa role of restricting, while getting elastically deformed, the threesets of spherical members 7 in the peripheral direction between the maleshaft 1 and the female shaft 2, and the three sets of cylindricalmembers 8 accommodated between the male shaft 1 and the female shaft 2perform a role of mainly transmitting the torque.

For example, when the torque is inputted from the male shaft 1, at aninitial stage, no backlash is caused due to the pre-load of the platesprings 9, and the plate springs 9 produce reaction to the torque, thustransmitting the torque. At this time, the torque is transmitted on thewhole in a state where a torque transmission load among the male shaft1, the plate springs 9, the spherical members 7 and the female shaft 1is equilibrated with a torque transmission load among the male shaft 1,the cylindrical members 8 and the female shaft 2.

Further, as a gap in the rotating direction between the male shaft 1 andthe female shaft 2 through the cylindrical member 8 is set smaller thana gap between the male shaft 1, the plate spring 9, the spherical member7 and the female shaft 2 through the spherical member 7, when the torqueincreases, the cylindrical members 8 receives the reaction stronger thanthe spherical members 7, and mainly the cylindrical members 8 transmitthe torque to the female shaft 2. It is therefore possible to surelyprevent the backlash in the rotating direction between the male shaft 1and the female shaft 2 and to transmit the torque in a state ofexhibiting the high rigidity.

Note that the spherical member 7 may be a ball. Further, the cylindricalmember 8 may be a needle roller.

The needle roller 8 receives the load in line contact and is thereforecapable of restraining a contact pressure lower than by the ball 7receiving the load in point contact, and has a variety of effects assuch. Accordingly, adopting the needle roller 8 is superior in terms ofthe following items to a case of applying a ball rolling structure toall the rows.

-   A damping capacity effect at the slide portion is larger than by the    ball rolling structure. Hence, high vibration absorption performance    is gained.-   If the same torque is transmitted, the needle roller can restrain    the contact pressure lower, whereby an axis-directional length can    be decreased, and the space can be effectively utilized.-   If the same torque is transmitted, the needle roller can restrain    the contact pressure lower, thereby eliminating the necessity for an    additional step of hardening the axis-directional groove surface of    the female shaft by a thermal treatment and so on.-   The number of components can be decreased.-   An assembling property can be improved.-   An assembling cost can be restrained.

Thus, the needle roller 8 performs a key role for transmitting thetorque between the male shaft 1 and the female shaft 2, and comes intoslide-contact with an inner peripheral face of the female shaft 2. Theneedle roller 8 has the following excellent points as compared with theconventional spline-fitting.

-   The needle roller is mass-produced and is therefore extremely low of    cost.-   The needle roller is polished after being subjected to the thermal    treatment and has therefore high surface hardness and an excellent    anti-abrasion property.-   As the needle roller is polished, its surface roughness is fine    enough to decrease the frictional coefficient when sliding, whereby    the slide load can be restrained low.-   A length and an arrangement of the needle roller can be changed    corresponding to a using condition, so that it is feasible to adapt    to a variety of applications without changing a design concept.-   There might be a case in which the frictional coefficient at the    sliding must be further decreased depending on the using condition.    At this time, the slide characteristic thereof can be altered simply    by effecting the surface treatment only on the needle roller, and    hence it is possible to adapt to the variety of applications without    changing the design concept.-   The needle rollers with different outside diameters can be    manufactured on a several micron basis at a low cost, and therefore    the gap between the male shaft, the needle roller and the female    shaft can be restrained down to the minimum by selecting a diameter    of the needle roller. Hence, improvement of the shaft rigidity in a    torsional direction is facilitated.

On the other hand, a structural point of partly adopting the balls hasthe following excellent items as compared with the structure in whichthe needle rollers are arranged in all the rows, and all the rows ofneedle rollers are slidable.

-   The frictional coefficient is low, and hence the slide load is    restrained low.-   The pre-load can be increased, and the prevention of the backlash    over a long period of time and the high rigidity can be attained    simultaneously.

FIG. 3 is a cross-sectional view taken along the line Y-Y in FIG. 1.FIG. 4 is a perspective view of the elastic members (the plate springs)connected by connecting portions. FIG. 5 is a view in the direction ofan arrow A shown in FIG. 1.

As shown in FIG. 1, an end portion of the male shaft 1 is formed with asmall-diameter portion 1 a. This small-diameter portion 1 a is providedwith a stopper plate 10 for regulating an axis-directional movement ofthe needle roller 8. This stopper plate 10 is constructed of an elasticmember 11 for an axis-directional preload, and a pair of flat plates 12,13 by which this axis-directional pre-load elastic member 11 is pinched.

Namely, according to the present embodiment, the flat plate 13, theaxis-directional pre-load elastic member 11 and the flat plate 12 arefitted in this sequence on the small-diameter portion 1 a, andsubsequently the end portion 1 b of the small-diameter portion 1 a iscaulked (or clinched), thus firmly fixing the stopper plate 10 to thesmall-diameter portion.

Note that a method of fixing the stopper plate 10 is not limited tocaulking (or clinching) and may involve using a locating snap ring, ascrew-fitting means, a bush nut and so on.

With this arrangement, the stopper plate 10 is constructed so that theflat plate 13 abuts on the needle rollers 8, and a proper pre-load canbe given to the needle rollers 8 from the axis-directional pre-loadelastic members 11 so as not to move in the axial direction.

The axis-directional pre-load elastic member 11 is made of rubber, resinor constructed of a plate spring made of steel sheet, and so forth. Theaxis-directional pre-load elastic member 11 may be separated from theflat plates 12, 13, however, it is preferable that the axis-directionalpre-load elastic member 11 and the flat plates 12, 13 be structured asan integral molding product in terms of considering easiness of theassembly.

For example, when the axis-directional pre-load elastic member 11 ismanufactured by use of the rubber, and, if manufactured byvulcanization-molding of the flat plates 12, 13, the integration thereofcan be attained, whereby an easy-to-assemble and low-cost product can bemanufactured.

Further, when the axis-directional pre-load elastic member 11 ismanufactured by using the resin, the integration can be attained byeffecting the integral molding of a corrugated elastic member 11 withthe flat plates 12, 13, wherein the same merit is obtained.

Moreover, the flat plates 12, 13 involve using the steel sheets, theresin or the steel sheets formed with a film of resin.

Furthermore, the grooves 3, 4, extending in the axial direction, of themale shaft 1 are terminated at axis-directional right-angled faces 14,15 substantially orthogonal to the axial direction and abutting on theball 7 and the needle roller 8.

As described above, the axis-directional movement, one its one side, ofthe needle roller 8 is regulated by the stopper plate 10 provided at thesmall-diameter portion 1 a of the male shaft 1, while, on the otherside, the axis-directional movement of the needle roller is regulated byits abutting on the axis-directional right-angled face 15.

Further, in the stopper plate 10, the flat plate 13 abuts on the needleroller 8, whereby the axis-directional pre-load elastic member 11 givesa proper pre-load to the needle roller 8 so as not to move in the axialdirection.

Accordingly, the fixation can be attained without any gap in the axialdirection by giving the proper pre-load to the needle roller 8. When themale shaft 1 and the female shaft 2 slide on each other, the needleroller is by no means moved in the axial direction, and an emission ofuncomfortable noises such as a butting sound can be surely prevented.

Moreover, the axis-directional grooves 3, 4 of the male shaft 1 have theaxis-directional right-angled faces 14, 15 substantially orthogonal tothe axial direction and abutting on the ball 7 and the needle roller 8.Therefore, with these axis-directional right-angled faces 14, 15, theaxis-directional movements of the ball 7 and the needle roller 8 can beregulated without providing special members. Consequently, themanufacturing cost can be reduced by decreasing the number ofcomponents, and besides it is possible to decrease a weight and to gaina compact configuration because of using none of the special members.

Next, according to the present embodiment, as shown in FIGS. 1, 3 and 4,the three pieces of plate springs 9 for giving the pre-load to the threesets of balls 7 are connected by a ring-shaped connecting portion 20. Tobe specific, as shown in FIG. 1, the small-diameter portion 1 a of theend portion of the male shaft 1 is formed with a stepped annular face21. The ring-shaped connecting portion 20 is fitted on thesmall-diameter portion 1 a and is disposed along the stepped annularface 21.

The stepped annular face 21 may take any configuration, etc on conditionthat the annular face 21 is an axis-directional annular face that facesin the axial direction of the male shaft 1.

The ring-shaped connecting portion 20 connects at three peripheral edgepoints thereof to the axis-directional ends of the three plate springs9. Namely, as shown in FIG. 4, the ring-shaped connecting portion 20 isconstructed integrally with the three plate springs 9 extending in theaxial direction. The three plate springs 9 may be different members.

Accordingly, though taking the complex structure of the balls 7 and theneedle rollers 8, the three plate springs 9 serving as the rolling facesare made integral, whereby the manufacturing cost can be reduced bydecreasing the number of the substantial components down to one fromthree, the decrease in the number of the components can be thusattained, and the assembling time is reduced by improving the assemblingproperty.

Moreover, the ring-shaped connecting portion 20 is not the circular-arcconnecting portion extending the peripheral direction as in the priorart, and can be therefore formed in the compact configuration withoutexpanding the diameter of the female shaft 2 in its diametricaldirection.

Still further, the small diameter portion 1 a formed at the end portionof the male shaft 1 penetrates the ring-shaped connecting portion 20.Accordingly, when assembling the three plate springs 9, the smalldiameter portion 1 a of the end portion of the male shaft 1 is insertedthrough the ring-shaped connecting portion 20, and thus performs afunction as a guide at this assembling time, thereby making it possibleto facilitate the assembling work and to reduce the manufacturing costby reducing the assembling time.

Yet further, the ring-shaped connecting portion 20 is disposed in theaxis-directional gap between the flat plate 13 of the stopper plate 10and the stepped annular face 21. This axis-directional gap is set on theorder of, e.g., 0.3 mm through 2.0 mm.

Owing to existence of this axis-directional gap, the ring-shapedconnecting portion 20 is, even when the three plate springs 9 getdeformed by an input of the torque, contrived not to restrict thesemotions of the plate springs 9.

Moreover, as shown in FIGS. 3 and 4, each of the plate springs 9 isformed in its sectional configuration taking a rectilinear shape that issubstantially parallel with the configuration of the axis-directionalgroove 3 of the male shaft 1. The plate spring 9 is constructed of aflat bottom portion 9 a at the center, first inclined side face portions9 b, 9 b extending divergently toward its outside diameter from bothends in the cross-sectional direction of the shaft with respect to theflat bottom portion 9 a, and second inclined face portions 9 c, 9 cfolded outward on the outermost diametrical sides of the first inclinedface portions 9 b, 9 b and extending substantially in parallel with thefirst inclined face portions 9 b, 9 b. A peripheral edge portion of thering-shaped connecting portion 20 is connected to the flat bottomportion 9 a of the central portion of each plate spring 9. The flatbottom portion 9 a of each plate spring 9 is press-fitted to the flatbottom portion 3 a of the groove 3, the second side face portions 9 c, 9c are press-fitted to the flat side face portions 3 b, 3 b of the groove3, and the first side plate portions 9 b, 9 b press the balls 7 and theneedle rollers 8 against the side faces of the grooves 5, 6 of thefemale shaft 2.

The small-diameter portion 1 a formed at the end portion of the maleshaft 1 penetrates the ring-shaped connecting portion 20. Adiameter-directional gap is formed between the small-diameter portion 1a of the male shaft 1 and the ring-shaped connecting portion 20. Thisdiameter-directional gap is on the order of, e.g., 0.2 mm through 1.0mm. As in the case of the axis-directional gap, owing to existence ofthis diameter-directional gap, the ring-shaped connecting portion 20 is,even when the three plate springs 9 get deformed by the input of thetorque, contrived not to restrict these motions of the plate springs 9.

Next, as shown in FIGS. 1 and 6, according to the present embodiment,the needle roller 8 is subjected to crowning. FIG. 6 is a side view ofthe cylindrical member (the needle roller) according to the embodimentof the present invention. A diameter of the needle roller 8 is graduallydecreased toward the end portions from the central portion in the axialdirection. A portion (φD1) having a largest diameter is the centralportion, while a portion (φD2) having a smallest diameter is a portionproximal to the end face (but excluding a radiused portion (R) formedwith the end face).

As shown in FIG. 6,

L1: an entire length of the needle roller,

L2: an axis-directional length subjected to the crowning,

L3: an axis-directional length of the maximum diameter portion,

φD1: a maximum diameter,

φD2: a minimum diameter, and

S: a difference quantity (a difference between the maximum diameter andthe minimum diameter with respect to a radius),

wherein a crowning dimensional relation be, it is desirable, set suchthat S=0.0003 mm through 0.500 mm and L2=L1×0.1 through L1×0.25.

Further, in the prior art, in the case of sliding the needle roller 8while being brought into contact with the male shaft 1 and the femaleshaft 2, the gap between the male shaft 1, the needle roller 8 and thefemale shaft 2 must be strictly controlled especially in the positionaldifference in the axial direction.

For example, if exhibiting such a tendency that the inside diameter ofthe female shaft 2 is gradually decreased towards an inner part from thevicinity of an entrance, the male shaft 1 comes to have an extremelylarge slide resistance at a point of time when the end face of theneedle roller 8 gains strong contact. If the entire gap is set large foravoiding this increase in the resistance, the periphery-directionalbacklash gets great.

By contrast, as in the present embodiment, the crowning portion enters atapered portion of the female shaft 2 by effecting the crowning on theneedle roller 8, and a longer slide stroke can be ensured whilerestraining the slide resistance low without any rise in the backlash inthe peripheral direction.

Thus, according to the present embodiment, the needle roller 8 issubjected to the crowning and gradually decreases in its diameter towardthe end portions from the central portion in the axial direction,whereby the needle rollers 8 of which the number is as small as possiblecan be disposed in one row of axis-directional grooves 4, 6.Accordingly, the assembling time can be decreased, and the needle rollercan be manufactured at the low cost.

Further, the needle roller 8 is subjected to the crowning, and it istherefore possible to, as described above, reduce the backlash in theperipheral direction while restraining the slide resistance low.

Moreover, as the needle roller 8 is subjected to the crowning, when thetorque is loaded, a gentle surface pressure is applied toward thecentral portion from the end portion of the needle roller 8. A localsurface pressure can be thus avoided, so that a lifetime of the productcan be kept long.

Note that the present invention is not limited to the embodimentdiscussed above and can be modified in a variety of forms.

As discussed above, according to the present invention, the cylindricalmembers of which the number is as small as possible can be disposed inone row of accommodating portions (the axis-directional grooves). Hence,the assembling time can be reduced, and the cylindrical member can bemanufactured at the low cost.

Moreover, the torque transmission member is capable of decreasing thebacklash in the peripheral direction while restraining the slideresistance low.

1. A telescopic shaft for vehicle steering which is assembled in asteering shaft of a vehicle and in which a male shaft and a female shaftare fitted for relative telescopic movement and for torque transmissiontherebetween, characterized in that: a respective cylindrical member isdisposed in at least one first set of accommodating portions beingformed by a first pair of axially extending grooves formed respectivelyon an outer peripheral surface of said male shaft and on an innerperipheral surface of said female shaft, said cylindrical member havingan axis that extends along an axial direction of said telescopic shaft,and said cylindrical member gradually decreasing in outer diameter froma central portion toward each of its end portions along the axialdirection; and a respective rolling member is disposed in at least onesecond set of accommodating portions being formed by a second pair ofaxially extending grooves formed respectively on the outer peripheralsurface of said male shaft and on the inner peripheral surface of saidfemale shaft, said rolling member being radially biased.
 2. A telescopicshaft for vehicle steering according to claim 1, wherein saidcylindrical member is crowned.
 3. A telescopic shaft for vehiclesteering according to claim 1, wherein the outer diameter of saidcylindrical member is tapered in a vicinity of each end portion of saidcylindrical member.
 4. A telescopic shaft for vehicle steering accordingto claim 1, wherein said cylindrical member is a needle roller.
 5. Atelescopic shaft for vehicle steering according to claim 1, wherein anaxial length of said cylindrical member, between said central portionand an end portion over which said outer diameter gradually decreases,is between 0.1 and 0.25 times an overall axial length of saidcylindrical member.
 6. A telescopic shaft for vehicle steering which isassembled in a steering shaft of a vehicle and in which a male shaft anda female shaft are fitted for relative telescopic movement and fortorque transmission therebetween, characterized in that: a respectivecylindrical member is disposed in at least one first set ofaccommodating portions being formed by a first pair of axially extendinggrooves formed respectively on an outer peripheral surface of said maleshaft and on an inner peripheral surface of said female shaft, saidcylindrical member having an axis that extends along an axial directionof said telescopic shaft, and said cylindrical member havingfrustoconical portions decreasing in outer diameter from a centralportion of said cylindrical member toward each of its end portions,respectively, along the axial direction; and a respective rolling memberis disposed in at least one second set of accommodating portions beingformed by a second pair of axially extending grooves formed respectivelyon the outer peripheral surface of said male shaft and on the innerperipheral surface of said female shaft, said rolling member beingradially biased.
 7. A telescopic shaft for vehicle steering according toclaim 6, wherein said end portions are radiused and each saidfrustoconical portion extends from said central portion to thecorresponding end portion.
 8. A telescopic shaft for vehicle steeringaccording to claim 6, wherein said cylindrical member is crowned.
 9. Atelescopic shaft for vehicle steering according to claim 6, wherein theouter diameter of said cylindrical member is tapered in a vicinity ofeach end portion of said cylindrical member.
 10. A telescopic shaft forvehicle steering according to claim 6, wherein said cylindrical memberis a needle roller.
 11. A telescopic shaft for vehicle steeringaccording to claim 6, wherein an axial length of each frustoconicalportion is between 0.1 and 0.25 times an overall axial length along saidaxis of said cylindrical member.